Gas-assist injection molds are similar to standard injection molds. Both single and multiple cavity molds can be made for gas-assist. Gas can be injected through the nozzle, in the runner, or in the article. If gas is injected through the nozzle of the machine, then mold design is similar to that of a standard injection mold. However, larger gates may be required to allow nitrogen to enter before the gate freezes off. Hot runner systems are not recommended when gas is injected through the machine nozzle and into the hot runner system, shot to shot variation and splay is likely. Hot runner systems require gas pins to be placed in the mold cavity and they also require a valve gate in order to stop the gas from entering the hot runner.

In-the-runner or in-the-article gas-assist requires the use of gas needles or pins. Standard gates can be used to direct the polymer melt into the thick regions of the gas channels, however jetting should be avoided. Gas needles need to be accessible from the face of the mold for cleaning purposes. Dismantling a mold to clean a clogged needle can lead to unnecessary down time and added work.

Gas-assist molds do offer some benefits over traditional injection molds. In some cases, hot runner systems can be eliminated when in-article runners or gas channels are used to promote material flow through the part. Since lower injection pressures are typically experienced, aluminum molds may be utilized. Aluminum molds can be machined easier, therefore reducing a mold's lead time and a products time to market. Steel molds though, are more robust and are sometimes necessary for high production runs, or when hydraulically actuated overflow cavities are considered. Overflow cavities with hydraulic slides can be used to enhance gas penetration into difficult areas. Tooling examples are contained in this section.